The discovery of T resident memory (TRM) cells in skin and other epithelial tissues has revolutionized our understanding of protective T cell memory. While we are delighted that our long standing R01 grant has contributed to the discovery TRM in humans, and the initial characterization skin TRM in mouse models of viral infection, these important cells remain poorly understood. We do know that TRM in skin provide much more rapid and potent protection against re-infection with pathogenic virus, and that TRM tend to not re-circulate from skin to blood. VACV skin infection results in the seeding of antigen specific TRM most effectively to infected skin, slightly less effectively to distant skin, and least effectively (but still significantly) to other epithelial organs that interface the environment (e.g., lung). Taken together, our murine and human data provoke us to make two related hypotheses: first, that these TRM are the most important T cells with regard to protective immune memory in general, and second, that their dysfunctional activation in man underlies most T cell mediated diseases of peripheral tissues, including skin (psoriasis), lung (asthma), gut (IBD), joint (RA), and CNS(MS). If these hypotheses are correct, then a better understanding of the fundamental biology of these TRM is central to our understanding of how to diagnose and more effectively treat a broad spectrum of human diseases, as well as to configure means to enhance protective immunity against pathogens. Using T cell receptor deep sequencing, we have made the unexpected observation that every TRM in tissue has a clonal counterpart in the lymph node TCM compartment, strongly arguing that TRM and TCM arise from a common naive T cell precursor. Thus, it is not antigen specificity per se, but rather their anatomic location, that makes TRM superior to TCM in terms of rapid protective immunity. The interplay between TRM and TCM, and the recruitment of existing TCM to become new TRM upon subsequent antigen challenge is also a focus of this grant. To broader our vantage point, we will use three distinct models of skin immune responses: contact hypersensitivity (CHS), vaccinia virus (VACV) infection, and C.albicans infection, coupled with genetically engineered mice, novel imaging approaches, and deep sequencing of the TCR, to try to better understand how TRM are generated and maintained, and what role they play in the maintenance of immune memory in viral, extracellular pathogen, and environmental antigen driven diseases. This grant is highly significant to human disease, uses innovative approaches and is based on novel technology, proposes ambitious but feasible approaches to be performed by experienced investigators in a supportive environment, and fervently hope it can be funded.